Microelectromechanical systems (MEMS) are systems employing electrical and mechanical technology that are fabricated using integrated circuit technology at microscopic dimensions. Micromachining processes are used to fabricate MEMS. Micromechanical devices and structures are likewise mechanical devices and structures fabricated using integrated circuit technology at a microscopic scale.
Electrically-actuated MEMS gimbaled mirrors have been designed for optical telecommunication switches for fiberoptic networks. Other MEMS devices have been used for other applications, including pressure and inertial sensing.
MEMS devices have employed electrostatic and capacitive circuits along with higher voltages, leading to problems involving shorting, leakage, parasitic capacitance, capacitive coupling, and reduced breakdown voltages, for example. Trench isolation techniques were developed to provide electrical isolation for MEMS devices to help to minimize such problems.
FIGS. 1A, 1B, and 1C illustrate a traditional trench isolation structure. Silicon beam 10 is comprised of two segments 12 and 14 separated by isolation joint 16. Isolation joint 16 is comprised of silicon dioxide (Si02), which is a dielectric. A layer 15 of silicon dioxide resides near the top of beam 10. A metal trace layer 11 resides over silicon dioxide layer 15 in order to provide desired electrical connections to particular portions of MEMS structures. Layers 17A and 17B reside on respective sidewalls of silicon beam 10. Layers of silicon dioxide also reside on respective sidewalls of isolation joint 16. The dielectric joint 16 provides a degree of electrical isolation between segments 12 and 14 of beam 16. Although segments 12 and 14 are electrically isolated, they are mechanically coupled via isolation joint 16.
According to one prior art technique, isolation joint 16 is formed by etching a trench in a silicon wafer and then oxidizing the silicon at high temperatures in an oxygen-rich environment. The oxidation process causes the silicon surface to form silicon dioxide. Silicon dioxide grows from the trench walls towards the center of the trench, called the nit line. Eventually the silicon dioxide growing from the two opposing side walls of the trench meets at the nit line, closing the trench opening. Beam 10 is formed by a silicon etch. Silicon dioxide is deposited on the sidewalls of beam 10 and isolation joint 16.
Sometimes degraded electrical isolation can occur with prior art trench isolation. FIG. 2A shows a cross section of silicon dioxide isolation joint 16 with silicon dioxide sidewalls 19A and 19B. Isolation joint 16 tapers near the top, shown as a slight bow shape in cross section. When the silicon wafer is etched, thick “stringers” of silicon 21A and 21B can result because of the shadowing affect of the oxide above the bowed region of isolation joint 16. Before releasing silicon beams from the substrate with an isotropic etch, sidewall oxide layers 19A and 19B are deposited. The sidewall oxide layers on isolation joint 16 protect the underlying structures from being destroyed during a release etch, but also shield the stringers from being etched. The stringers of silicon sometimes wrap around the sides of isolation joint 16. The stringers of silicon are conductive, so they can form a parasitic conductive path around isolation joint 16.
Any exposed silicon on the oxide isolation joint 16 away from the oxide sidewalls is etched during the release etch. Therefore, silicon stringers sometimes go around the sides of isolation joint 16, but generally not under the bottom of isolation joint 16.
FIG. 2B shows a variation of the problem, with thin stringers of silicon 23A and 23B on isolation joint 16. Stringers 23A and 23B are generated from an inductively coupled plasma (“ICP”) etch process. Because of alternating deposition-etch procedures, small silicon scallops 23A and 23B can result, each one being a stringer.
Because of the stringer problem, when voltages above 50 volts are used with certain prior art trench isolation, the electrical isolation can degrade and allow currents on the order of microamps. When trench isolation is used on a gimbaled MEMS mirror that is part of an array of MEMS mirrors in an optical telecommunications switch, that level of electrical conduction can be sufficient to disrupt the operation of the mirror array. Degraded electrical isolation can be especially problematic at voltages above 100 volts for a MEMS mirror array.